First you say to do, and then you don’t, and then you say you will, and then you won’t,” begins an old pop song called “Undecided.”

Some staffers at the Centers for Disease Control and Prevention (CDC) — if any are old enough — might have been quietly humming that tune on Monday, thinking about their own beleaguered agency.

That’s because the CDC’s posted guidelines for dealing with the Covid-19 coronavirus were changed on Friday to reflect most scientists’ views on how the virus moves from one human host to another — but then on Monday, the agency reversed course.

Undecided, indeed.

The agency said the Friday post was an early, unapproved draft, “posted in error.”

After months of study, most outside experts now say the coronavirus most commonly spreads via respiratory droplets, or aerosols, so tiny they can hang suspended in the air much longer, and drift much farther, than previously thought.

And briefly, over the weekend, that’s what the CDC said, too.

Aerosol particles can cause infection when “inhaled into the nose, mouth, airways, and lungs,” the CDC said in the revised guidelines posted on Friday. “This is thought to be the main way the virus spreads.

But on Monday, the agency changed its recommendations back to the way they were last week, saying Covid-19 is thought to spread mainly between people in close contact — 6 feet or less — “through respiratory droplets produced when an infected person coughs, sneezes or talks,” says CNN, which first reported the story on Sunday.

Coughing, sneezing or talking produces droplets much larger than aerosols, which don’t travel far and remain in the air for a much shorter time.

The edited Web page has removed all references to airborne spread, except for a disclaimer that recommendations based on this [aerosol] mode of transmission are under review,” says the Washington Post.

“We are returning to the earlier version and revisiting that process. It was a failure of process at CDC.” said Jay Butler, the agency’s deputy director for infectious diseases.

It’s unclear if the replacement of new guidelines for old was ordered by CDC officials, or others higher up in the Trump administration. The president has been unwavering in his oft-stated belief that masks, for example, do little to protect people from Covid-19. He rarely wears one.

But virtually all experts still say wearing a mask in public places is one of the most effective ways to protect others — and to a limited extent, yourself — from the virus.

In a separated but possibly related development, Trump-appointed Health and Human Services officials recently pushed to change the language of weekly CDC science reports released by the CDC “so as not to undermine Trump’s political message,” says CNN, citing an unnamed federal health official.